Heh. Not used to games not having overlays, but if it is that early on then within reason.
If it does not appear to have compression then chances are it does not (you can try compressing it with one of said same tools and seeing what size differences happen -- a compressed file tends not to compress again. Not all games used such things -- the DS uses fairly slow clunky memory that was cheap even at the time and a few kilobytes did not make a lot of odds.
script.dat in the assembly viewer is very unlikely to yield anything useful.
In the hex window the numbers at the start when flipped 0041, 0204, 0303, 0517, 0620,....) look to be counting upwards and keeping reasonably low in number, which says maybe that is file level pointers of some form (for that kind of length probably paragraph or section if that is indeed the game's text -- script.dat is by no means a sacred name or anything and could be whatever the devs wanted, or even nothing at all). Equally that many runs of 0000 would be something I expect compression algos to make very short work of which is another in the "not compressed" camp.
Anyway text script wise if you are editing an English language game then I would probably bust out the relative search tool ( https://www.romhacking.net/utilities/513/
) and use that on the ROM to search for either a line from the game or sometimes I get lazy and do " the " without the quotes as most scripts will feature such a phrase. If it is compressed or non relative then it might not show anything but still worth a try.
If you wanted to scroll down to a bit later in the script.dat file (crystaltile2 will note what file the cursor corresponds to in the bottom status bar) to I am guessing about 0204 in from the start (though could go for 00DFA400 + 0303 and see what is there*) and maybe mess around with the file a bit (swap some values around, overwrite a section with another value you see there) and then run the game to see what changed then that might be an idea (this is a method called corruption wherein you mess up a file and see what happens in the game, whatever changes happened in the game are then a result of your efforts and hopefully you noted where those efforts were or compare between your modded ROM and stock ROM to see)
*it might be nothing, it might be for another file, it might be offset (some things count from the start of the section, not the start of the file) or relative.